The present invention relates generally to tape drives, and more particularly to an apparatus which positions a magnetic read/write head relative to a length of magnetic tape. The disclosure of commonly assigned U.S. patent application Ser. No. 849,140 now abandoned filed Apr. 7, 1986 and entitled "Cartridge Loading Apparatus", is hereby incorporated by reference.
Many small business and personal computer systems utilize backup memory devices in the event of a malfunction in the principal memory device of the computer. Previously, floppy disk drive units have been used to store data in such an event. As an alternative to floppy disk drives, streaming tape drives have been developed as a means for storing data on a length of 1/4" wide magnetic tape which is wound within a tape cartridge. As a backup memory device, tape drives are preferable to floppy disk drives because tape drives have a much greater storage capacity, and can load data onto a computer much more quickly.
To store as much data as possible on the magnetic tape, the data is recorded on a number of parallel, longitudinally oriented tracks on the tape. In order to read and write the data onto the tape in separate tracks, the magnetic head must be able to move transversely across the tape in increments as small as the width of a track, and be maintained in the proper orientation at each track. Typically, a led screw driven by a stepper motor is provided for this purpose. The head is mounted on a positioning apparatus which threads onto the lead screw so that for a given amout of rotation of the lead screw, the head is displaced a predetermined distance across the tape. Alternatively, the motor may cause the lead screw to move in a linear fashion, as opposed to rotating. In such a case, the lead screw is not necessarily threaded onto the head positioning apparatus.
One way to increase the data storage capacity of the tape is to divide the tape into more tracks which are smaller in width. As a result, the head must be positioned with even greater precision. Thus, the clearance between the head positioning apparatus and the lead screw must be minimized to prevent motion of the head other than when the lead screw is in motion. Previously, springs have been provided to bias the head positioning apparatus relative to the lead screw to eliminate any clearance therebetween.
However, the head positioning apparatus is still subject to motion due to disturbance which may vibrationally excite the apparatus and cause the head to resonate. Most notably, the apparatus will be vibrated by the motor, which often overshoots and causes the lead screw to oscillate about a desired position. The resulting vibration will be of an amplitude large enough to affect the desired precision of the orientation of the head.
The spring will load the motor by providing resistance to motion of the head, which helps to dampen vibration of the head positioning apparatus. Since the load on the motor is a function of the spring force, if the spring force were to remain constant at a predetermined level, the precise amount of damping could be provided to cause the head to reach an equilibrium position quickly and with a minimum of oscillation. However, as the head is moved, the spring is deflected and the spring force varies. This is because the spring force is generally proportional to the degree of deflection of the spring. Thus, the spring force and its damping characteristics will vary as the head moves, potentially subjecting the head to significant vibration.
Further, it is desirable for the head positioning apparatus to occupy as little space as possible. This enables the external dimensions of the tape drive to be minimized, so that the tape drive can fit within a standard size enclosure within a computer. For example, a tape drive manufactured in accordance with the "51/4 inch half height form factor" set forth by ANSI must have a width of no more than 53/4 inches, a height of no more than 15/8 inches, and a depth of no more than 8 inches. Unfortunately, as the spring is reduced in size to save space, the spring is more likely to behave in a "non-linear" fashion. That is, the spring force will vary to a greater degree for a given amount of deflection of the spring. As a result, it becomes even more difficult to maintain a constant load on the motor which will dampen vibrations.
Thus, a need exists for a compact head positioning apparatus which dampens vibration and allows precise positioning of a magnetic head so that multiple tracks of data may be recorded on a length of magnetic tape.